How to handle 'design X from scratch' when you've never used X?

i recently got asked to design Spotify from scratch in an interview, and while I use the service daily, I realized I had no idea about the internals of audio streaming or content delivery networks at that scale. i felt like i was fumbling through it, relying on general distributed systems knowledge but lacking domain-specific insights. how do you approach 'design X from scratch' questions when X is a domain you've never personally worked in? are there specific frameworks for reasoning about unfamiliar territory, or strategies to demonstrate problem-solving skills even if you don't know the specific solutions?